Exposure to even low levels of arsenic in drinking water and food may increase the risk of developing, and dying from, heart disease, a new study suggests.

In the study, researchers analyzed urine samples from 3,575 American Indians in Arizona, Oklahoma and North and South Dakota, living in regions where arsenic levels in drinking water were "low to moderate," meaning they were above the limit set by the U.S. Environmental Protection Agency, which is 10 micrograms per liter, but below 100 micrograms per liter.

Urine samples were collected between 1989 and 1991, and the participants were divided into four groups based on the concentration of inorganic arsenic in their urine. (There are two types of arsenic, organic and inorganic, inorganic arsenic is thought to be more toxic).

The participants were followed until 2008, and those in the group with the highest urine arsenic concentrations were 32 percent more likely to develop cardiovascular disease, and 65 percent more likely to die from the condition over the nearly 20-year period, compared to the people in the group with the lowest arsenic urine concentrations. [7 Foods You Can Overdose On]

The findings held after the researchers took into account certain risk factors for heart disease, such as smoking and high body mass index. However, the strength of the link declined slightly after the researchers took into account other risk factors: high blood pressure, diabetes and liver disease, suggesting that these factors may explain part of the link between arsenic exposure and cardiovascular disease risk, the researchers said.

While previous studies have linked high levels of arsenic exposure to increased risk of cancer and heart disease, few studies have looked at the effects of low to moderate exposure.

In 2001, the EPA estimated that the drinking water of 13 million Americans had arsenic levels above the limit of 10 micrograms per liter. People can also consume the element through their food, although no limit has been set for food.

Recently, the Food and Drug Administration set a limit for the amount of arsenic in apple juice, but has not done so for food. Concerns have been raised over levels in rice, but after testing, the agency said that levels of arsenic in rice were too low to pose health risks over the short term, although it said more research on the long-term health effects is needed.

"Given the large population exposed, even a modest increased risk of cardiovascular disease due to arsenic could have important public health implications," the researchers wrote in Monday's (Sept. 23) issue of the journal Annals of Internal Medicine. "These findings support the importance of low to moderate arsenic exposure as a cardiovascular risk factor with no apparent threshold," they said.

Because the study involved American Indian populations, who are at an increased risk for diabetes, the findings would be particularly applicable to other groups that also have a high diabetes risk, said study researcher Dr. Ana Navas-Acien, an associate professor of environmental health sciences and epidemiology at Johns Hopkins Bloomberg School of Public Health.

The results are based on a single measurement of urine arsenic levels, which took place at the study's start, so it's not clear if the study participants' levels would have remained the same over time. However, a separate analysis done by the researchers showed that the measurement taken at the study's start was generally consistent with measurements taken at two other points.

It's not clear how arsenic exposure might increase the risk of heart disease, and the study cannot prove that arsenic exposure causes heart disease. Diabetes, high blood pressure and liver disease — which were more common among those with the highest urine arsenic concentrations — could play a role.

It could also be that some other factor, related to both arsenic exposure and cardiovascular disease risk, could explain the link, Navas-Acien said. Studies in animals suggest that arsenic exposure increases the risk of developing plaques in the arteries, or arteriosclerosis, which can cause heart disease.

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Rachael Rettner

Rachael has been with Live Science since 2010. She has a masters degree in journalism from New York University's Science, Health and Environmental Reporting Program. She also holds a Bachelor of Science in molecular biology and a Master of Science in biology from the University of California, San Diego.